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Retina horizontal cells
Horizontal cells are the laterally interconnecting neurons in the outer plexiform layer of the retina of mammalian eyes. They help integrate and regulate the input from multiple photoreceptor cells. Among their functions, horizontal cells are responsible for allowing eyes to adjust to see well under both bright and dim light conditions. Organization There are three basic types of horizontal cells, designated HI, HII and HIII. The selectivity of these three horizontal cells, towards one of the three cone types, is a matter of debate. According to studies conducted by Boycott and Wassle neither HI cells nor HII cells were selective towards S,M, or L cones. By contrast, Anhelt and Kolb claim that in their observations HI cells connected to all three cone types indiscremenantly, however, HII cells tended to contact S cones the most. They also identified a third type of horizontal cell, HIII, which was identical to HI but did not make contact with S cones. The HII cells also make connections with rods, but do so far enough away from the horizontal cell's soma such that they do not interfere with the activities of the cones. They span across cones and summate inputs from them all to control the amount of GABA released back onto the photoreceptor cells, which hyperpolarises them. Their arrangement together with the on-center and off-center bipolar cells that receive input from the photoreceptors constitutes a form of lateral inhibition, increasing spatial resolution at the expense of some information on absolute intensity. The eye is thus more sensitive to contrast and differences in intensity. Functional Properties When light is shown onto a photoreceptor, the photoreceptor hyperpolarizes and reduces the release of glutamate, when this happens, horizontal cells reduce the release of GABA, which has an inhibitory affect on the photoreceptors. This reduction of inhibition leads to a depolarization of the photoreceptors. We therefore have the following negative feedback Illumination \to photoreceptor hyperpolarization \to horizontal cell hyperpolarization \to photoreceptor depolarization One proposed theory for facilitation by the horizontal cells proceeds as follows. Assume we have 10 photoreceptors, one hyperpolarizing (H) bipolar cell, and one horizontal cell. All ten photoreceptors connect to the horizontal cell, and the middle photoreceptor ( P_m ) connects to the bipolar cell. The surrounding cells, which represent the outer receptive field, will be designated P_o then we can explain an off-centre arrangement as follows. If light is shown onto the P_m then # P_m is activated by light and therefore hyperpolarizes # P_m reduces release of glutamate #Reduction of glutamate hyperpolarizes the H bipolar cell #Reduction of glutamate hyperpolarizes the horizontal cell and it reduces release of GABA #Since P_o is still releasing glutamate, reduction in GABA is marginal If the light is shone onto the surrounding area then # P_o is activated and therefore hyperpolarizes # P_o reduce release of glutamate #Reduction of glutamate hyperpolarizes the horizontal cell #Horizontal cell reduces release of GABA #Reduction of GABA depolarizes photoreceptors # P_o not affected since they are strongly being hyperpolarized by activation # P_m is affected and therefore depolarizes # P_m releases glutamate #H Bipolar cell is depolarized To explain diffuse light, then we consider both cases together, and as it turns out, the two effects cancel each other out, and we get little or no net effect. See also *Photoreceptors * Bipolar cells *Amacrine cells *Ganglion cells External links *Webvision horizontal cell entry. References * * * Category:Eye